It's different with your own car, but I always abide by one rule when I drive other people's vehicles: Never drive at more than 50%. Yes, this may seem quite conservative, but the last thing I want to do is damage another man or woman's pride and joy. Even if it's a rental.
This guy in the following video doesn't seem to run by the same rule and pushes his rented Ferrari 458 110% on a canyon road. This can, and will, only end one way.
I really insist that you live by my rule when driving other cars. Not only for the fact tha you could lose someone their car, but for that fact that you'll end up paying out a lot of money for a quick thrill that will last five seconds but drag on for months.